Manufacture of yarn is carried out as a continuous operation, and as part of the manufacturing process, machines have been developed to automatically package yarn by winding it onto bobbins for storage, shipment, and sale.
In a typical example, one such winding machine has a mandrel which rotates at a selectable speed, and onto which an empty bobbin or core is placed. Such mandrels may grip the bobbin with teeth, rubber collars, expandable and retractable "dogs", or O-rings. Various adjustments are then possible to alter the angle at which the yarn is wound, the tension on the yarn during winding, and the speed at which the bobbin is spun. Yarn is then wound onto the bobbin, preferably in a uniform pattern, until the desired quantity of yarn is loaded, and the full bobbin is then removed from the mandrel and replaced with an empty one.
Each such bobbin is thus required to withstand compressive forces occasioned by the winding of the yarn, the high-speed rotation of the bobbin, and the centrifugal forces created in the bobbin. Customarily, many such bobbins are cone-shaped, and the stresses incurred during winding are complex because the bobbin does not have a uniform cross-sectional configuration throughout its length.
Accordingly, bobbins manufactured for use with automatic winding machines must be strong enough structurally to withstand the rotational stresses induced during winding and must facilitate insertion onto and removal from the mandrel of the winding machine. As winding machines of different manufacturers vary in their constructions, so mandrel configurations and constructions differ, and the bobbin should preferably be constructed for use with a variety of mandrels.
Another important consideration is maintaining a close fit between the bobbin and the mandrel which will limit slippage, yet which will allow easy removal of the bobbin from the mandrel when winding is complete.
It has also been shown to be advantageous to have some form of guiding structure formed on the bobbin's outer surface to direct the first layer of yarn to be wound uniformly along the bobbin, making it possible for tight, uniform winding of yarn which maximizes the amount of yarn carried on each bobbin, and facilitates dispensing of the yarn by the ultimate user. Past efforts in this area have included forming a series of concentric raised rings on the surface of the cone in order to grip the yarn during the first stages of winding. Another prior attempt to attract and direct the winding of the yarn is to "stipple" the surface of the bobbin with small, raised "dots". Typically, the surface of the stippled bobbin has a sand paperlike feel, and prolonged handling of such bobbins is likely to abrade the user's hands unless gloves are worn.